The present invention relates to an ion guide, a mass spectrometer, a method of guiding ions and a method of mass spectrometry.
It is well known that the time averaged force on a charged particle or ion due to an AC inhomogeneous electric field is such as to accelerate the charged particle or ion to a region where the electric field is weaker. A minimum in the electric field is commonly referred to as a pseudo-potential well or valley. Correspondingly, a maximum is commonly referred to as a pseudo-potential hill or barrier. RF ion guides are designed to exploit this phenomenon by causing a pseudo-potential well to be formed along the central axis of the ion guide so that ions are confined radially within the ion guide.
Different forms of AC or RF ion guide are known including those constructed using multi-pole rod sets, for example quadrupole, hexapole and octapole rod sets. Also known are ion tunnel or stacked ring ion guides which comprise a stacked ring electrode set wherein opposite phases of an AC or RF voltage are applied to adjacent electrodes. A further known ion guide comprises a series of diametrically opposed AC or RF plate electrodes with DC top and bottom plates, otherwise known as a sandwich-plate ion guide.
A quadrupole rod set ion guide generates a radially symmetric quadrupolar field. To obtain a perfect field it is necessary for the rods to have a hyperbolic cross section. Other types of rod may be used to approximate a quadrupolar field. For example, circular rods, concave rods and flat rods may be used. Quadrupole rod sets are often used for analytical devices such as quadrupole mass filters, linear ion traps and other similar devices. However, their restricted stable mass range and poor acceptance can restrict their use as an ion transport device.
Ion tunnel ion guides have a wide mass range and their flat bottomed/steep sided pseudo-potential leads to good acceptance and transmission characteristics.
It is desired to provided an improved ion guide.